1. Field of the Invention
This invention relates to the inhibition of oxidation in lubricants. More particularly, the invention concerns lubricants in which have been placed an additive capable of reducing oxidation thereof.
2. Discussion of the Prior Art
Lubricants, such as oils and greases, are subject to oxidative deterioration at elevated temperatures or upon prolonged exposure to the elements. Such deterioration is evidenced, in many instances, by an increase in acidity and in viscosity, and when the deterioration is severe enough, it can cause metal parts to corrode. Additionally, severe oxidation leads to a loss of lubrication properties of the lubricants, and, in especially severe cases, to complete breakdown of the device being lubricated. In combatting oxidation, many additives have been tried, but many of them are only marginally effective except at high concentration, especially when the lubricant is subjected to drastic oxidizing conditions.
No references are known that disclose or suggest the additives of this invention. It will be shown herein that the unesterified hindered hydroquinones are not effective. Tending to confirm this is U.S. Pat. No. 3,095,287, which discloses that a similar hindered phenol (4-methyl-2,6-di-tert-butyl phenol) fails to provide thermal stability in jet fuels.
The compounds are useful in a variety of lubricants. Those which may be improved by adding the hindered hydroquinone thereto are mineral and synthesized lubricating oils, as well as greases made therefrom. The mineral oils will be understood to embrace not only the paraffinic, but also the naphthenic and aromatic-containing members. By synthesized oils are meant synthesized hydrocarbons, polyalkylene oxide oils, polyacetals, polysilicones and the like, as well as synthetic ester oils. Of the latter type there may be mentioned esters made from monohydric alcohols and polycarboxylic acids, such as 2-ethylhexyl azelate and the like, and those made from polyhydric alcohols and aliphatic monocarboxylic acids. Those of this group are especially important, and they include esters prepared from the polymethylols, as for example, the trimethylols, such as ethane, propane and butane derivatives thereof, 2,2-disubstituted propane diols and the pentaerythritols with aliphatic monocarboxylic acids containing from about 4 to about 9 carbon atoms. Mixtures of these acids may be used to prepare the esters. Preferred in the practice of this invention are the esters prepared from a pentaerythritol and a mixture of C.sub.5 -C.sub.9 acids. In making such esters, a generally acceptable product can be made from commercial pentaerythritol containing about 88% monopentaerythritol and 12% dipentaerythritol.
Having described the invention in general terms, the following is offered as a specific embodiment thereof. It will be understood that the example is merely for the purpose of illustration, and there is no intention to limit the scope of the invention to the member shown.